1. A new rat model has been developed allowing body fluid status to be accurately controlled and maintained throughout experimentation by computer-driven, servo-controlled replacement of spontaneous urinary fluid losses. 2. Experiments in vitro were performed to test the accuracy of the servo system, and experiments in vivo were carried out to re-assess basic renal function in servo-controlled vasopressin-replete Long Evans and vasopressin-deficient Brattleboro rats. The model was further evaluated in water-diuretic Wistar rats with or without administration of a vasopressin V 2 -receptor agonist, 1-desamino-8- d -arginine vasopressin. 3. The data gained from the present study indicate the suitability of the servo-controlled replacement system for conscious renal function studies in three different rat strains. Haemodynamic and renal function variables measured were demonstrated to be stable throughout a 5 h experimental procedure and reproducible between repeated experimental occasions over a 14 day post-operative period. 4. Using the servo-control technique, the expected action of 1-desamino-8- d -arginine vasopressin on renal water handling was demonstrated, but the natriuretic effect reported by some workers was not evident. 5. Since the servo-controlled fluid replacement technique maintains many of the inherent differences between vasopressin-replete Long Evans and vasopressin-deficient Brattleboro rats and eliminates the changes in body fluid volume during transition from a diuretic to an antidiuretic state, the model confers an advantage over previously employed constant infusion protocols.